1. Technical Field of the Invention
This invention pertains to improved resource allocation and response time in trunked radio communication systems.
2. Description of the Prior Art
A trunked radio communication system is one in which a number of radio users (subscribers) share a group of communication channels, which is practical when each user requires a channel only a small percentage of the time and few will be communicating simultaneously. Subscribers may use mobile or portable two-way radio units and fixed base dispatch consoles. Typical systems use radio repeaters to provide the communication channels. A repeater receives inbound transmissions from subscriber units on a first path and rebroadcasts the transmissions at higher power on a second, outbound path destined to other units in the field. Typical trunking systems use different assigned frequencies for these simultaneous paths. Each pair of inbound and outbound frequencies makes up a single communication channel, of which a trunked system has several. Channels carrying voice and other message communications are known as information channels; those carrying signalling and control information are known as control channels. These channels commonly involve simplex communication: that is, alternate, rather than simultaneous, transmission and reception.
A trunked system typically requires that a user desiring to communicate first request permission to access the group of channels and then wait for permission and for an assignment to a particular channel. The trunked system provides means for receiving requests and for transmitting assignments, often by digital signalling on the control channel. If the system operates under the discipline of transmission-trunking, the user retains his channel assignment for only a single transmission; if the system uses message-trunking, the user retains his assignment until completing an entire message comprising several back-and-forth transmissions. Upon completing the transmission or message, respectively, the subscriber relinquishes the channel to the pool of resources for use by others. Because trunked systems involve sharing resources, subscribers must often wait for availability of information channels, in which case the trunk controller responds to a request for channel with a busy signal and notifies the subscriber when a channel later becomes available. U.S. Pat. No. 4,635,285, Communication System with Voice Priority for Remote Stations, issued Jan. 6, 1987, to Coombes and incorporated herein by reference, provides details of trunked system operation and signalling protocols.
Trunked radio systems generally provide dispatch communication and may include capability for wireline interconnection. Dispatch communication is the delivery of messages from a dispatcher to subscribers and the reception of responses from subscribers. It is common for the trunked system to treat all communication positions, i.e., dispatcher and subscribers, equivalently. To make a transmission, a dispatcher or subscriber will place a request for a channel, wait for a grant of permission and an assignment to a channel, and begin to transmit on the assigned channel. After receiving a transmission, a subscriber who wishes to respond will, in turn, begin transmission on an assigned channel. Communication takes place by transmission from a talking subscriber to the central location on an inbound frequency and repeated transmission to listening subscribers on the paired outbound frequency.
Wireline interconnection usually involves coupling a simplex radio subscriber on the trunked system with a subscriber of a duplex wireline telephone network. The reader is referred to U.S. Pat. No. 4,677,656, Telephone-Radio Interconnect System, issued Jun. 30, 1987, to Burke, et al., and incorporated herein by reference, for details of interconnect communication and call origination.
A trunked interconnect system following prior art practices will simultaneously assign inbound and outbound frequencies and will maintain the assignments for the duration of a conversation. The radio subscriber transmits under push-to-talk control on the inbound frequency; information received at the repeater couples to the wireline network instead of repeating on the paired outbound frequency. Signals from the wireline subscriber couple to the outbound repeater for broadcast to the radio subscriber; these signals may, but need not, key the repeater by voice-operated control. The duplex wireline subscriber can always hear and may, therefore, be interrupted by the radio subscriber. While transmitting, the simplex radio subscriber does not hear, and cannot be interrupted by, the wireline party.
Access to the wireline network from a trunked system requires a call origination procedure. One often used is for a radio subscriber to place a request for interconnect service along with his request for a channel. When a channel capable of supporting interconnect service becomes available, the controller assigns and grants permission to use it. The radio subscriber originates the wireline call by transmitting tone or data signalling information (e.g., DTMF dual-tone multi-frequency or packet data encoded telephone number), which couples to the wireline network. The system transmits the call progress information to the radio subscriber on the paired outbound frequency. The radio subscriber hears busy tones or ringing tones and eventually becomes connected with the wireline party or terminates his call attempt.
Several features of prior art trunked systems have proven objectionable. First, when the system is heavily loaded with radio traffic, subscribers wishing to transmit must wait for vacant channels, which often involves receiving a busy signal and waiting for a call-back with permission to transmit. Upon receiving the call-back announcement, the requesting subscriber may be given a short response time in which to seize the assigned channel. Subscribers experience access delays, and the response time for channel seizure costs valuable communications time.
Second, prior art systems assign inbound and outbound frequencies together for interconnect traffic, even though the simplex radio subscriber cannot make use of them simultaneously. Interconnect conversations are generally more protracted than dispatch traffic, so unnecessarily assigning paired channels removes valuable resources from the pool available to interconnect calls.
Third, the call origination procedures taught by the prior art consume valuable information channel time that might be used for other information communications.